SysMLv1: A 'part property' in SysMLv1 does not formally include 'port' (which is considered a special kind of property), even though the Port metaclass in UML2 extends Property and a 'port' may have 'composite' aggregation and be typed by a Block.

In SysMLv1, a part property (a type of block property) is a specific concept, refined from the UML2.5.1 concept of a 'part object', and distinct from 'port'. Formally, a 'port' in SysML is not a 'part property', even when typed by a Block and with AggregationKind 'composite'.

However, the SysMLv1.7 spec is in some spots contradictory if taken literally:

SysMLv1.7: 'Four general categories of properties of blocks are recognized in SysML: parts, references, value properties, and constraint properties.'

The following would seem to imply that a 'port' is not "general" in the sense used above:

SysMLv1.7: 'Ports are specialized kinds of properties, and can be shown in same way as other properties. They can appear in block compartments in the same format as other properties of their owning blocks, or as the ends of associations, with the port appearing ...'

The above is slightly misleading, since the notations for a 'port' in an IBD are different from those for a 'part property', as shown in the IBD diagram above.

Additionally, one may show a Port on the boundary of a Block symbol in a BDD, although this is not recommended Webel Best Practice:

Webel: SysMLv1: Prefer not show Port symbols on the boundary of Block symbols in BDDs, just list them in compartments. One can't connect them in a BDD anyway. Recommend only show Port symbols in IBDs.

Also, the following, if taken literally, could pedantically be interpreted as including a 'port' typed by a Block as a 'part property'!

SysMLv1.7: 'A property typed by a SysML Block that has composite aggregation is classified as a part property, except for the special case of a constraint property ...'

Consider a port, with AggregationKind 'composite', and typed by a Block. Is it a 'property'? Well, Port extends Property. And SysMLv1.7 says "Ports are specialized kinds of properties". So, perhaps yes, and if so, then that 'port' typed by a Block by the definition above is also a 'part property'!

Confusion can arise where the term 'property' is used. In some cases in the SysMLv1 specification it means strictly a Property (a specific metaclass), and excludes extending metaclasses such as Port; in some other cases the 'term' property is used to mean a Property OR any extending metaclass (so includes Port).

Note, however, that a 'port' MAY be typed by a ValueType, even if it is not recommended for SysMLv1.7:

SysMLv1.7: A port MAY be typed by a ValueType (and validates in Cameo Systems Modeler) but this is NOT recommended best practice. Prefer a Block or InterfaceBlock with a directed FlowProperty typed by a ValueType.

Use of a ValueType as the type of a 'port' is a legacy approach associated with the DEPRECATED FlowPort and the DEPRECATED concept of atomic flow port (as opposed to a nonatomic flow port typed by the DEPRECATED FlowSpecification.

A direct Property (not a Port) typed by a ValueType is a value property, not a part property.

In UML, a 'port' is also described separately from the concept of 'part object':

UML-2.5.1: 'Composite aggregation is a strong form of aggregation that requires a part object be included in at most one composite object at a time. If a composite object is deleted, all of its part instances that are objects are deleted with it.'

It's quite clear on the the notations being different, too:

SysMLv1.7: 'A part may be shown by graphical nesting of a box symbol '

SysMLv1.7: 'A Port of an EncapsulatedClassifier is shown as a small square symbol. The Port symbol may be placed either overlapping the boundary of the rectangle symbol denoting that EncapsulatedClassifier or it may be shown inside the rectangle symbol.'

The option above for the port to 'be shown inside the rectangle symbol' is supported in Magic Cyber-Systems Engineer^® (Cameo Systems Modeler^®) for SysMLv1 ports; try setting the visibility to 'private'.

If you are inside an IBD in Cameo and you use the Refactor > Convert To to switch between a Port and a Part Property you'll see that it correctly jumps from other being displayed as a Port symbol on the boundary or as a Part symbol completely inside its owning Block.

This aggregation rule applies to both 'port' and 'part':

UML2.5.1: 'A Port is a Property of an EncapsulatedClassifier that specifies a distinct interaction point between that EncapsulatedClassifier and its environment or between the Behavior of the EncapsulatedClassifier and its internal roles. Ports are connected by ...'

Even though the UML Port metaclass extends the UML Property metaclass, it is not considered a part in the sense described above.

One thing that can be a bit confusing is that in SysMLv1 there is no actual stereotype PartProperty in SysMLv1. The Magic Cyber-Systems Engineer^® (Cameo Systems Modeler^®) tool has a dedicated extension PartProperty, which it uses as part of enforcing the rules described for the SysMLv1 'part property', but that's a tool implementation concern (and not inconsistent with the SysMLv1 spec).

Relates to

Boundary Ports and kinds of Connectors

Related notes